1. Field of the Invention
The present invention relates to a multi-user (MU) transmission method and multiple input multiple output (MIMO) transmission system using the same, and scheduling method and access point (AP) using the same, more particularly, to a MU transmission method and MIMO transmission system using the same, and scheduling method and AP using the same capable of scheduling simultaneous transmission to a plurality of stations.
2. Description of the Prior Art
Wireless local are network (WLAN) is widely used in our daily life to provide access to the internet with mobile devices. To improve the throughput in WLAN, IEEE 802.11n standard adopts a multiple input multiple output (MIMO) system that transmits a plurality of streams with multiple antennas and, at the same time, receives the streams with multiple antennas. However, IEEE 802.11n is still based on a point-to-point transmission scheme. When there are more stations connected to an access point (AP), each station (namely user) has to hold the transmission and wait for an empty time slot, i.e. only one station can link with the AP at one time slot.
Carrier sense multiple access/collision avoidance (CSMA/CA) is one of the most popular media access control protocol in wireless communications. For example, in IEEE 802.11 standard, CSMA/CA is defined to achieve wireless channel access while “request to send” (RTS) and “clear to send” (CTS) are used to avoid the hidden terminal problem, i.e. data collision may be caused by multiple stations transmitting data to the AP at the same time. That is, when the AP intends to transmit data to a station, the AP first sends an RTS to the station. Once receiving the RTS, the station responds to the AP with a CTS. After receiving the CTS, the AP starts transmitting data. During this period, other stations monitoring the CTS or RTS should refrain from sending data. In other words, conventionally, only one station can access the channel in one RTS and CTS interval.
In order to increase data throughput, a multi-user (MU) transmission system with MIMO system is proposed. A MU-MIMO system can transmit data to multiple stations with multiple antennas or single antenna at the same time, such that more stations can be served by the AP at the same time.
However, when two stations simultaneously link with the AP, data transmitted in two channels corresponding to the two stations may be interfered by each other if channel conditions of the two channels are similar, i.e. not orthogonal to each other enough. Therefore, the media access control (MAC) used in a conventional MU-MIMO system is performed on a schedule base. In other words, the AP arranges the schedule (time slots), frequency and antenna to transmit data to different stations, i.e. still only one station can link with the AP at one time slot, and does not use CSMA and RTS/CTS packets. Thus, there is a need for improvement of the prior art.